Showing posts sorted by relevance for query BASICS OF HRCT INTERPRETATION-PART 2- RETICULAR PATTERN. Sort by date Show all posts
Showing posts sorted by relevance for query BASICS OF HRCT INTERPRETATION-PART 2- RETICULAR PATTERN. Sort by date Show all posts

BASICS OF HRCT INTERPRETATION

In this article a practical approach is given for the interpretation of HRCT examinations

We will discuss the following subjects:
  • Anatomy of the secondary lobule
  • Basic HRCT patterns
  • Distribution of abnormalities
  • Differential diagnosis of interstitial lung diseases
 Secondary lobule




 Secondary lobule
Knowledge of the lung anatomy is essential for understanding HRCT.

  • The secondary lobule is the basic anatomic unit of pulmonary structure and function.
  • Interpretation of interstitial lung diseases is based on the type of involvement of the secondary lobule.
  • It is the smallest lung unit that is surrounded by connective tissue septa.
  • It measures about 1-2 cm and is made up of 5-15 pulmonary acini, that contain the alveoli for gas exchange.
  • The secondary lobule is supplied by a small bronchiole (terminal bronchiole) in the center, that is parallelled by the centrilobular artery.
  • Pulmonary veins and lymphatics run in the periphery of the lobule within the interlobular septa.
  • Under normal conditions only a few of these very thin septa will be seen.
  • There are two lymphatic systems: a central network, that runs along the bronchovascular bundle towards the centre of the lobule and a peripheral network, that is located within the interlobular septa and along the pleural linings.


Centrilobular area is the central part of the secondary lobule.
It is usually the site of diseases, that enter the lung through the airways ( i.e. hypersensitivity pneumonitis, respiratory bronchiolitis, centrilobular emphysema ).
Perilymphatic area is the peripheral part of the secondary lobule.
It is usually the site of diseases, that are located in the lymphatics of in the interlobular septa ( i.e. sarcoid, lymphangitic carcinomatosis, pulmonary edema).
These diseases are usually also located in the central network of lymphatics that surround the bronchovascular bundle.


Basic Interpretation
A structured approach to interpretation of HRCT involves the following questions:
  • What is the dominant HR-pattern:
    • reticular
    • nodular
    • high attenuation (ground-glass, consolidation)
    • low attenuation (emphysema, cystic)
  • Where is it located within the secondary lobule (centrilobular, perilymphatic or random)
  • Is there an upper versus lower zone or a central versus peripheral predominance
  • Are there additional findings (pleural fluid, lymphadenopathy, traction bronchiectasis).
 These morphologic findings have to be combined with the history of the patient and important
clinical findings.

  • When we study patients with HRCT, we have to realize that we are looking at a selected group of patients.
  • Common diseases like pneumonias, pulmonary emboli, cardiogenic edema and lung carcinoma are already ruled out.
  • So uncommon diseases like Sarcoidosis, Hypersensitivity pneumonitis, Langerhans cell histiocytosis, Lymphangitic carcinomatosis, Usual Interstitial Pneumonitis (UIP) and many others become regular HRCT diagnoses.
Read my next article in the series


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BASICS OF HRCT PART 6 - DISTRIBUTION PATTERN AND ADDITIONAL FEATURES

Upper lung zone preference is seen in:
  • Inhaled particles: pneumoconiosis (silica or coal)
  • Smoking related diseases (centrilobular emphysema
  • Respiratory bronchiolitis (RB-ILD)
  • Langerhans cell histiocytosis
  • Hypersensitivity pneumonitis
  • Sarcoidosis
Lower zone preference is seen in:
  • UIP
  • Aspiration
  • Pulmonary edema
Central distribution is seen in sarcoidosis and cardiogenic pulmonary edema.

Peripheral distribution is mainly seen in cryptogenic organizing pneumonia (COP), chronic eosinophilic pneumonia and UIP.
Additional findings
Pleural effusion is seen in:

1.     Pulmonary edema
2.     Lymphangitic spread of carcinoma - often unilateral
3.     Tuberculosis
4.     Lymphangiomyomatosis (LAM)
5.     Asbestosis

Hilar and mediastinal lymphadenopathy

In sarcoidosis the common pattern is right paratracheal and bilateral hilar adenopathy ('1-2-3-sign').
In lung carcinoma and lymphangitic carcinomatosis adenopathy is usually unilateral.
'Eggshell calcification' in lymph nodes commonly occurs in patients with silicosis and coal-worker's pneumoconiosis and is sometimes seen in sarcoidosis, postirradiation Hodgkin disease, blastomycosis and scleroderma .

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BASICS OF HRCT INTERPRETATION-PART 3- NODULAR PATTERN

Nodular pattern

The distribution of nodules shown on HRCT is the most important factor in making an accurate diagnosis in the nodular pattern.
In most cases small nodules can be placed into one of three categories: perilymphatic, centrilobular or random distribution.
Random refers to no preference for a specific location in the secondary lobule.
Perilymphatic distribution
In patients with a perilymphatic distribution, nodules are seen in relation to pleural surfaces, interlobular septa and the peribronchovascular interstitium.
Nodules are almost always visible in a subpleural location, particularly in relation to the fissures.
Centrilobular distribution
In certain diseases, nodules are limited to the centrilobular region.
Unlike perilymphatic and random nodules, centrilobular nodules spare the pleural surfaces. 
The most peripheral nodules are centered 5-10mm from fissures or the pleural surface.
Random distribution
Nodules are randomly distributed relative to structures of the lung and secondary lobule. 
Nodules can usually be seen to involve the pleural surfaces and fissures, but lack the subpleural predominance often seen in patients with a perilymphatic distribution.
Algorithm for nodular pattern

The algorithm to distinguish perilymphatic, random and centrilobular nodules is the following:
  • Look for the presence of pleural nodules.
    These are often easiest to see along the fissures.
    If pleural nodules are absent or few in number, the distribution is likely centrilobular.
  • If pleural nodules are visible, the pattern is either random (miliary) or perilymphatic.
  • If there are pleural nodules and also nodules along the central bronchovascular interstitium and along interlobular septa, you are dealing with a periplymphatic distribution.
  • If the nodules are diffuse and uniformly distributed, it is likely a random distribution.
Perilymphatic distribution

 Perilymphatic nodules are most commonly seen in sarcoidosis.
They also occur in silicosis, coal-worker's pneumoconiosis and lymphangitic spread of carcinoma.
Notice the overlap in differential diagnosis of perilymphatic nodules and the nodular septal thickening in the reticular pattern.
Sometimes the term reticulonodular is used.
Here a typical case of perilymphatic distribution of nodules in a patient with sarcoidosis.

Notice the nodules along the fissures indicating a perilymphatic distribution (red arrows).
Always look carefully for these nodules in the subpleural region and along the fissures, because this finding is very specific for sarcoidosis.
Typically in sarcoidosis is an upper lobe and perihilar predominance and in this case we see the majority of nodules located along the bronchovascular bundle (yellow arrow).
Another typical case of sarcoidosis.

In addition to the perilymphatic nodules, there are multiple enlarged lymph nodes, which is also typical for sarcoidosis.
In end stage sarcoidosis we will see fibrosis, which is also predominantly located in the upper lobes and perihilar.

Centrilobular distribution

 Centrilobular nodules are seen in:
  • Hypersensitivity pneumonitis
  • Respiratory bronchiolitis in smokers
  • infectious airways diseases (endobronchial spread of tuberculosis or nontuberculous mycobacteria, bronchopneumonia)
  • Uncommon in bronchioloalveolar carcinoma, pulmonary edema, vasculitis


In many cases centrilobular nodules are of ground glass density and ill defined (figure).
They are called acinair nodules.

Tree-in-bud

In centrilobular nodules the recognition of 'tree-in-bud' is of value for narrowing the differential diagnosis.
Tree-in-bud describes the appearance of an irregular and often nodular branching structure, most easily identified in the lung periphery. 
It represents dilated and impacted (mucus or pus-filled) centrilobular bronchioles.
Tree-in-bud almost always indicates the presence of:

  • Endobronchial spread of infection (TB, MAC, any bacterial bronchopneumonia)
  • Airway disease associated with infection (cystic fibrosis, bronchiectasis)
  • less often, an airway disease associated primarily with mucus retention (allergic bronchopulmonary aspergillosis, asthma).
Here a tree-in-bud is seen.

In the proper clinical setting suspect active endobronchial spread of TB.
In most patients with active tuberculosis, the HRCT shows evidence of bronchogenic spread of disease even before bacteriologic results are available (6).
Random distribution

HRCT of a patient with random nodules as a result of miliary TB.
The random distribution is a result of the hematogenous spread of the infection.
Small random nodules are seen in:
  • Hematogenous metastases
  • Miliary tuberculosis
  • Miliary fungal infections
  • Sarcoidosis may mimick this pattern, when very extensive
  • Langerhans cell histiocytosis (early nodular stage)
Sarcoidosis usually has a perilymphatic distribution, but when it is very extensive, it spreads along the bronchovascular bundle to the periphery of the lung and may reach the centrilobular area.

Langerhans cell histiocytosis is an uncommon disease characterised by multiple cysts in patients with nicotine abuse.
In a very early stage, these patients show only nodules, that later on cavitate and become cysts (figure).

As in all smoking related diseases, there is an upper lobe predominance.




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Air Bronchogram Sign.

Pulmonary Medicine Blog By Dr Deepu

Air bronchogram

An air bronchogram is a tubular outline of an airway made visible by filling of the surrounding alveoli by fluid or inflammatory exudates. It is almost always caused by a pathologic airspace/alveolar process, in which something other than air fills the alveoli.

 Six causes of air bronchograms are
Lung consolidation
Pulmonary edema
Nonobstructive pulmonary atelectasis
Severe interstitial disease
Neoplasm
Normal expiration.
In The Image Air Bronchogram is seen.
The Magnified Image Showing Air Bronchogram On CXR and Confirmed With HRCT


The phenomenon is Characterised by  air-filled bronchi (dark) being made visible by the opacification of surrounding alveoli (grey/white). It is almost always caused by a pathologic airspace/alveolar process, in which something other than air fills the alveoli.
 Air bronchograms will not be visible if the bronchi themselves are opacified (e.g. by fluid) and thus indicate patent proximal airways.
Air bronchograms that persist for weeks despite appropriate antimicrobial therapy should raise the suspicion of a neoplastic process. CT may be planned in such cases.

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The ‘Dark Bronchus’ Sign: For diagnosis of PCP

Pulmonary Medicine Blog By Dr Deepu

Today I will discuss the importance of the ‘dark bronchus’ sign in the diagnosis of uniform, diffuse ground glass opacification on high resolution computerized tomography (HRCT). This sign is useful to identify diffuse ground glass opacity on HRCT in cases of Pneumocystis carinii pneumonia who may present with a normal or equivocal chest radiograph in the early course of disease.

Chest radiograph is the initial investigation in HIV patients with chest symptoms. But even in patients with proven PCP, radiographic findings may be normal in up to 20-40%. Low incidence of PCP in patients with normal or equivocal findings on chest radiograph despite high clinical suspicion emphasizes the need for a noninvasive and widely available investigation in such cases.

Various modalities to investigate symptomatic HIV patients with normal, equivocal or nonspecific radiographic findings include carbon monoxide diffusion in lung (DLCO), gallium citrate lung scanning and HRCT. A DLCO of less than 80% of the predicted value has a sensitivity of up to 98% for PCP, but the specificity is less than 50% and the measurement is not always available. Although gallium scanning has a sensitivity of up to 100% for PCP in patients with abnormal radiographs, it has never been prospectively studied in patients with normal or equivocal radiographic findings. In addition, this investigation requires a 48- to 72-hour delay in imaging, is not readily available and has a high cost.



    On the other hand, HRCT is a widely available and noninvasive investigation for PCP. Patchy or diffuse ground glass opacity is the most frequent finding. Other findings include cystic changes (33%), centrilobular nodules (25%), lymphadenopathy (25%) and pleural effusion (17%). HRCT has been found to be especially important in the assessment of symptomatic patients with normal, equivocal or nonspecific radiographs. In such cases, it shows high sensitivity (100%), specificity (86%) and accuracy (90%) for PCP, using only the presence or absence of ground glass opacity as the criterion for positivity.
The Arrow Shows The Dark Bronchus Relative to The Surrounding Lungs

Patchy ground opacity or mosaic attenuation, which is observed in up to 92% of the patients, can be easily identified on HRCT. However, subtle ground glass opacification, especially when bilateral and diffuse, may be difficult to diagnose. This is because of bilateral uniform increase in lung attenuation with absence of normal lung parenchyma for comparison. In such cases, the ‘dark bronchus’ appearance is a useful sign to identify diffuse ground glass opacity. This finding refers to the presence of air-filled bronchi appearing ‘too black’ relative to the surrounding lung parenchyma, which is filled with inflammatory alveolar exudates. This subtle finding may help in identification of patients with ‘possible PCP’ despite a normal or equivocal chest radiograph. Subsequently direct test for PCP (i.e., broncho-alveolar lavage) may be initiated for definitive diagnosis and treatment.
Hence the importance of the ‘dark bronchus’ sign in the diagnosis of uniform, diffuse ground glass opacification on HRCT. This is especially useful in the presence of a normal chest radiograph and ‘near normal’ HRCT. HRCT offers an accurate and early diagnosis in patients with normal chest radiographs; it alters patient management and facilitates early therapy.

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